High speed switch devices are used in various technologies, including, for example, automotive safety systems and microwave relay systems. In each of these systems, switch devices must operate reliably and at high frequencies.
In a vehicle safety system, for example, inertia-operated mechanical switches often are used to sense the onset of a vehicle crash event and trigger the actuation of an occupant protection device (e.g., an air bag or a seat belt pretensioner). More recently, vehicles are being equipped with one or more electronic acceleration sensors working in cooperation with a microprocessor and sophisticated software. The sensors provide electronic signals proportional to sensed vehicle acceleration. The microprocessor assesses the changes in the vehicle's acceleration to determine whether the signal indicates that a vehicle crash event is in progress that requires actuation of the occupant protection system.
Safing switches often are used in combination with acceleration sensors to provide a redundant level of detection for a vehicle crash event. The safing switch usually is designed and calibrated to close at a relatively early stage in a crash event. The occupant protection device (e.g., an air bag or seat belt pretensioner) is only actuated when the safing switch is closed and the microprocessor determines that the severity of the crash is sufficient to warrant such actuation.
Safing switches may be manufactured by processes generally similar to those used to assemble other small mechanical devices. Efforts are being made, however, to develop smaller switches that could be manufactured using techniques like those used to manufacture semiconductor components and/or micro-machined silicon elements. Patents describing such micro-machined switch devices include U.S. Pat. Nos. 5,331,853 and 5,591,910.
U.S. Pat. No. 5,331,853 discloses an acceleration sensor micro-machined from a silicon substrate. The sensor includes pair of accelerometers, each having a force sensing axis. Each accelerometer includes a mass supported by micro-machined flexures connected to a support frame so as to permit movement of the mass relative to the substrate. Each accelerometer measures movement of. the associated mass so as to provide an output signal indicative of the sensed acceleration along its force-sensing axis.
U.S. Pat. No. 5,591,910 discloses a micro-machined acceleration sensor formed of an inertial mass supported above a substrate by flexure hinges. The inertial mass moves relative to the substrate when subjected to an acceleration perpendicular to the plane of the substrate. Movement of the mass results in a change in a capacitance value of the sensor. The changing capacitance value is indicative of acceleration.
Other examples of micro-machined switch devices and accelerometers are disclosed in U.S. Pat. Nos. 4,736,629; 4,882,933; 5,541,437; 5,635,739; and 5,804,783.